1. Technical Field of the Invention
The present invention relates generally to the nondestructive evaluation of thermal properties and more particularly to a method and apparatus for quantitatively and thermographically analyzing a structure for disbonds and/or inclusions.
2. Description of the Prior Art
The use of laminated structures in aerospace applications has increased in recent years. It has been demonstrated that thermographic techniques such as laser scanners, infrared (IR) cameras and contact temperature sensors such as thermocouples for inspection of these structures for disbonds has significant advantage in portability, expense and adaptability to field use compared to other methods. Many of these thermal techniques use full field noncontacting imaging and accordingly require a significant amount of equipment, making the inspection limited to areas of easy access. Many of the test structures currently in use are located in a somewhat inaccessible area, or their geometry is such that it is impractical to attempt to use a full field IR imager for inspection.
U.S. Pat. Nos. 4,647,220 and 4,854,724 to Adams et al disclose IR analysis of corrosion spots and spot welds. The test area is heated with a pulsed IR heat input and problem areas identified after cooling as localized hot spots, i.e., a relative measure is obtained with respect to surrounding areas rather than a quantitative value related to thermal properties over time. The technique of Adams et al is unable to recognize imperfections which encompass the entire heated area since it relies on a comparative determination.
U.S. Pat. No. 4,950,990 to Moulder et al discloses a system for photoinductive imaging for material flaw detection. A laser heats a specified area of the material and a probe detects the resulting eddy currents which are indicative of flaws or holes.